Alvin Plantinga, echoing a worry of Charles Darwin which he calls 'Darwin's doubt', argues that given Darwinian evolutionary theory our beliefs are unreliable, since they are determined to be what they are by evolutionary pressures and could have had no other content. This papers surveys in turn deterministic and non-deterministic interpretations of Darwinism, and concludes that Plantinga's argument poses a problem for the former alone and not for the latter. Some parallel problems arise for the Cognitive Science of Religion, and in particular for the hypothesis that many of our beliefs, including religious beliefs, are due to a Hypersensitive Agency-Detection Device, at least if this hypothesis is held in a deterministic form. In a non-deterministic form, however, its operation need not cast doubt on the rationality or reliability of the relevant beliefs